A circulator is a device with three or more ports where a microwave or RF signal is transferred from one port to the next in a prescribed order. A variety of circulators are known. However, some of these include active circulators. Typical active circulator consists of active transistors and each source (or emitter) of the transistors are tied together (common node).
Prior arts suffer from DC power loss leading to less efficiency, significant insertion loss and port isolation issues. FIG. 1 shows a prior art three port active circulator. The port numbers are labeled and as can be seen, the direction of transfer is clockwise. In this prior art circulator, the drain bias voltages (Vdd) are directly connected to three drain resistors and the drain currents flow thought these drain resistors. The common node, where the sources of the transistors are tied together, is grounded to Vss through common a ground resistor. There are significant voltage drops through these various resistors causing DC power consumption to occur making these circulators less energy efficient. If these drain resistors and common source resistors are not of high enough value, there can be significant RF power loss as well. The voltage drops across these resistors also limit the maximum RF voltage swing across each transistor, thus limiting the power handling capacity of these circulators.
Also in FIG. 1, there are three other resistors bypassed with the capacitors in parallel (R1-C1, R2-C2, R3-C3)) which form RF feedback networks. These RF feedback networks are typically used in an active circulator in order to adjust transconductance of the transistors. The active circulator operation is very sensitive to RF transconductance and it affects circulator loss and isolation parameters and the resulting performance.
An improved active circulator is disclosed in application Ser. No. 14/554,995 as shown in FIG. 2. In order to improve the power efficiency, and increase the power handling capacity of the circulator, drain resistors are eliminated in FIG. 2. In order to make a circulator operational, its common source node has to be connected to the ground through the resistor (Rc) or resistor and inductor in parallel (Rc and Lc in parallel) as shown in FIG. 2. Under the small signal operation, the common node voltage is almost a constant, since common node current flowing doesn't change that much. But, once a large signal is applied, common node current keep increasing over the power level and corresponding common node voltage keeps rising (DC or AC). In an active circulator, the control of various biases is critical. In this case, each transistor gate bias has to be set around unit current gain point to make closed loop circulator stable. Once this low gate bias point moves off from the original point by the power increase (common node voltage change), the active circulator will lose the circulation and isolation. Typically, large signal power increases the common node voltage and effective gate-source voltage (Vgs) of each transistor becomes lower than the original setting. Effective drain to source voltage (Vds) will also experience the same by the large signal. These changes in Vgs and Vds worsen the circulator loss and isolation parameters, affecting the circulator performance.
Circulators have numerous uses. For example, one port of a three port circulator may be connected to an antenna, while a receiver is connected to a second port of the circulator to receive signals received by the antenna and with a transmitter connected to a third port of the circulator to supply transmit signals to the antenna (with the transmit signals being isolated by the circulator from the receiver which might otherwise be damaged by the transmit signals).